1. Field of the Invention
The present invention relates to apparatus used for the cooling of air. More specifically, the present invention relates to apparatus for cooling air through indirect evaporative cooling and heat exchange.
2. Related Art
1. Evaporative Cooling
One method of cooling air is referred to as evaporative cooling. Evaporative cooling is the process of cooling dry air with water. As the air and water are brought into contact with one another, heat from within the air is transferred to the water as the water vaporizes into a gaseous mixture with the air. Usually, as in an evaporative cooler, this is achieved by pulling dry, warm air through a wet fiber mat, thereby creating moist, cool air. There are two major drawbacks to this form of coolingxe2x80x94first, the air is only cooled to a temperature well above dew point (saturation) temperature due to a short residence time, and second, evaporative cooling increases the humidity of the cooled air. This increased humidity results in discomfort, which can also create health and equipment problems within the space to be cooled.
2. Indirect Evaporative Cooling
A related and newer method of cooling air is referred to as xe2x80x9cindirect evaporative cooling.xe2x80x9d Indirect evaporative cooling is the process of indirectly cooling ambient dry outdoor air, prior to its entering a space to be xe2x80x9cair-conditionedxe2x80x9d or xe2x80x9cconditioned,xe2x80x9d with saturated, cool air that is outdoor air that has passed though an evaporative cooler with a long residence time. This is typically done by passing the two bodies of air through air passages adjacent to one another. This process results in heat transfer from the warmer, dry air body, into the cooler, saturated air body, thereby resulting in the cooling of the dry air body without increasing its moisture content. The apparatus housing such adjacent air passages is often referred to as an xe2x80x9cindirect heat exchangerxe2x80x9d or xe2x80x9cindirect cooling module.xe2x80x9d
The major drawback to the use of indirect evaporative cooling has been cost. Currently, manufactured indirect cooling modules are expensive, mainly due to the fact that they are being made in molded layers specifically-designed as indirect cooling modules. Therefore, what is needed is a source of material for indirect cooling modules that is in common supply and is inexpensive relative to the cost of specifically-molded indirect cooling modules.
A second drawback to conventional indirect evaporative cooling has been efficiency. Conventionally-manufactured indirect cooling modules are inefficient, mainly due to the fact that effective evaporative cooling takes place only in the top half of the conventional modules. There are two main reasons for this: poor water distribution and poor air distribution across the inlet face. In addition, inefficiency results from the dry air side not having sufficient heat transfer surface area above the cool, saturated air side.
The water distribution problem occurs due to the construction of the conventional modules. In the prior art, cooling air is passed vertically upward while water is discharged, from a header at the top of the module, downward against the up-flow of the cooling air. In doing this, some of the water evaporates, thereby cooling the air flowing upward while the unevaporated water drops to a reservoir below to be pumped back to the top. In conventional cooling modules, effective evaporative cooling takes place only at the upper half of the module, thereby limiting the cooling efficiency.
The air distribution problem arises in trying to get the air passing through adjacent chambers within the module to have the proper turbulence, also known as minimizing the boundary layer and maximizing residence time. Even distribution of air across the face of the module is the key to gaining proper turbulence, yet none of the currently-used modules have been able to properly solve this problem. Therefore, what is needed is an indirect cooling module which ensures even distribution of air across the face of the module, and therefore achieves the maximum heat transfer while providing the least amount of pressure drop and air contact velocity.
An example of one indirect evaporative cooler is shown in U.S. Pat. No. 5,664,433 (Bourne et al.) which discloses an xe2x80x9cindirect and direct evaporative cooling system.xe2x80x9d The Bourne et al. cooler uses specifically-designed heat exchange plates and a double cooling system where the air is first cooled through indirect cooling and then further cooled using direct cooling. The Bourne et al. device uses an indirect heat exchanger manufactured by the Adobe Corporation of Phoenix, Ariz. under two U.S. Patents (U.S. Pat. Nos. 4,566,290 and 4,461,733). These patents (""290 and ""733) disclose a thin-wall formed and molded plate system, with an added felt coating on the evaporatively cooled air side. The Bourne et al. indirect heat exchanger has an efficiency rating of less than 40%, thus relying on the direct cooling stage to create a majority of the cooling effect.
Another patent, U.S. Pat. No. 4,512,392 (van Ee et al.), discloses a heat exchange apparatus using a heat exchanger core formed from polypropylene extruded sheets. The van Ee device differs from the present invention in a number of ways, and lacks many of the efficiency-increasing features of the present invention. The van Ee patent discloses an air-to-air heat recovery system, rather than an air cooling system using evaporatively-cooled air to refrigerate incoming air. The van Ee device uses short spacers to separate opposing sheets, but gives them no other function. The van Ee patent precludes the use of the heat exchanger coupled with additional cooling means, while the present invention may include a direct evaporative cooling module, vapor-compression coils and multi-staged indirects. Finally, the van Ee patent is designed primarily for livestock with controls set for air flushing of mold, bacteria, etc, while the present invention is primarily designed for human comfort.
What is still needed, therefore, is an indirect evaporative cooling apparatus that is inexpensive and is more efficient than conventional indirect evaporative cooling modules, due to improvement of water and air distribution within the apparatus.
The present invention is an indirect evaporative cooling module that accomplishes heat exchange between fluids in its adjacent primary and secondary passages. More specifically, the invented cooling module features counter-current contact of a liquid and a secondary fluid to evaporatively cool the secondary fluid. The module also features a flow of primary fluid, substantially isolated from the liquid and the secondary fluid, in passages that are separate from, but adjacent to, the secondary fluid passages. Heat exchange takes place between the secondary fluid and the primary fluid, and is preferably optimized by a number of efficiency-increasing features, mainly focusing on improving flow distribution in the module, increasing turbulence of the cooled air down to the molecular boundary layer, and improving contact between liquid and fluid in the secondary passages. These features may include: 1) roughening of the surfaces of the secondary fluid passages to increase surface area of the secondary air passages and improve heat transfer; 2) down-comer channels that isolate some liquid from the secondary fluid to direct that portion of liquid to the bottom of the heat exchange module for splashing or other mixing with the incoming secondary fluid; and/or 3) a pre-cooling chamber for increased liquid-secondary fluid contact located up-stream of the secondary passages, in the secondary fluid stream at or near the secondary fluid inlet. Thus, the invented cooling module maximizes heat exchange in a given module size by using substantially all of the available surface area for heat exchange rather than approximately the top half.
Also, the efficiency-increasing features may include turning vanes, convergers, or other flow management members at the fluid inlets and/or at the fluid outlets. For example, turning vanes may be positioned near the primary fluid inlet of the heat exchanger for directing the primary fluid into the primary passages with a more even flow distribution. Convergers may be positioned at the bottom of the heat exchanger near the secondary fluid inlet to direct the secondary fluid into the secondary passages with a more even flow distribution.
Additional improvement of fluid-liquid contact in the invented cooling module is done by improved management of the water used in the cooling process, and by a high surface area evaporative pad or a mesh for retention of the water in the secondary passages. The preferred water distributor at the top of the heat exchanger module efficiently and evenly directs water into all of the secondary passages and all of the down-comers, without plugging, to provide moisture consistently on substantially the entire secondary surface. The water distributor creates an efficient top evaporative cooling zone, while the down-comer system described above creates an effective lower evaporative cooling zone, both top and bottom zones substantially inside the passages of the module. The mesh is preferably positioned inside each secondary passages from the top of the passages to the bottom of the passages, to distribute and retain water both from the distributor and from the down-comers for contact with and evaporation into the up-flowing air.
To create yet another cooling zone, mesh or other high surface area pad preferably is also positioned to hang outside of, and just below, the inlet of the secondary passages. This creates a pre-cooling chamber directly below the cooling module corrugated sheets, inside the module housing, which gives the system a xe2x80x9chead-startxe2x80x9d in cooling the secondary air.
The invented heat exchanger cooling module preferably cooperates with, on the inlet side, a primary air source, a secondary air source, and a water source. On the outlet side, the heat exchanger cooperates with a water recovery and/or recirculation system, a cooled primary air outlet system for distributing cooled air to the space being air-conditioned, and a secondary air outlet system for handling the wet air exiting the heat exchanger. These inlet and outlet systems may be of conventional construction and operation.
The indirect cooling module preferably is made from extruded, twin-walled, corrugated, fluted plastic sheeting. This sheeting is an inexpensive, commercially available material, which greatly reduces the cost of such a cooling module. Sheets of this corrugated material are cut into plates and strips which are preferably laminated together to form the primary and secondary passages of the heat exchanger core. These passages can be configured any number of ways, ranging in position from vertical to horizontal to diagonal.
The internal passages formed by the corrugations inside each sheet become the primary air passages, while space between adjacent sheets of the corrugated material become the secondary passages. Typically, such corrugated sheets include a fluted or rippled patten on their exterior surfaces, which are formed when the internal corrugations are formed. These flutes aid in creating turbulence in the secondary passages and therefore aid the secondary air to xe2x80x9cscrubxe2x80x9d heat from the fluted passage walls. The flow through the primary passages and secondary passages may be cross-flow, counter-flow and even baffle-flow, relative to each other, as long as consideration is made for proper design of the piping in the inlet and outlet systems to maintain the primary and secondary streams separate. The corrugated plates are preferably cut into rectangular, trapezoidal or rhombus configurations, to form a core adapted to the size and shape of the cooler in which the core is installed, and adapted to the inlet and outlet piping of the particular cooler.
Preferably, the strips that are laminated between sheets of the corrugated material are positioned to extend substantially all the way from the top area of the corrugated sheets to the bottom area of the corrugated sheets, with their internal passages run generally vertically. This way the strips may serve a dual purpose: 1) to separate opposing sheets to create the space between sheets that becomes the secondary passages, and 2) to have an interior channel(s) that channel(s) water from the water distribution system to near the bottom of the sheets, particularly to near the inlet of the secondary passages, to contact the incoming secondary air near the bottom of the passages and, preferably to wet the mesh in the pre-cooling chamber. Thus, by using an inexpensive preformed material, one may cut and install down-comer xe2x80x9ctubesxe2x80x9d that also form an important sheet spacing function. The spacer strips are preferably cut to include more than one channel, and preferably installed in at least two positions along the width of the sheets, near each edge of the sheets. Alternatively, other tubes may be used as down-comers rather than the corrugated strips. For example, extruded members that have one or more cavities per member may be used, as long as the cavity is adapted for conveying the liquid through the tube.